1. Field of the Invention
This invention relates to a very low cost magnetic recording apparatus and method, and in particular to an apparatus and. method for high speed recording of video signals.
2. Description Relative to the Prior Art
The search for high speed, efficient reproduction of magnetic tape cassettes is driven by the continually growing market for home viewing movies. Home movie watching, which is almost exclusively serviced by VHS machines, requires an ongoing stream of movie titles recorded on magnetic tape cassettes. The bulk of the market is tape rentals, and to satisfy the voracious appetite of this market, considerable effort has been expended in attempts to speed up the reproduction of movie cassette tapes by use of higher tape speeds during the reproducing process.
A limited number of systems is known in the art for writing videotaped information at speeds greater than the normal playback speeds. For example, U.S. Pat. No. 4,872,070 to Cooper et al. describes a system and method for high speed videotape reproduction. The source of the video information for reproduction is from a laser disc player which is specially equipped with two optical read heads. The optical disc player also rotates the disc at twice the standard speed allowing information to be read from the optical disc at four times its standard playback rate. In a similar fashion, the helical scan videotape recorder unit has two write head pairs for writing the video information on the videotape in parallel while the tape is moved at twice its normal speed thus producing a 4xc3x97 write capability. This system lacks the ability to write videotapes at higher than four times their normal viewing speed and requires that the video information come from a video disc player. Although two parallel video signal paths are described, the system lacks the ability to decompress video images from a compressed video file format.
Another system known in the art for writing videotape at a speed faster than the normal viewing speed is found in U.S. Pat. No. 5,065,258 to Warren et al. This system is an analog to analog system in which an analog videotape is played at a higher than normal speed and the signals therefrom are recorded by a second video recorder at the same higher than normal viewing speed. This system lacks the ability to write videotapes at more than twice the normal viewing speed and lacks the ability to process parallel streams of video data from a compressed file format.
Another type of video duplication device is shown in U.S. Pat. No. 5,260,800 to Sturm et al. This system duplicates video cassette tapes at twice the normal viewing speed. The source video information comes from modified video disc players which operate at twice the normal NTSC video format speed. The video and audio information is converted to analog signals which are written at twice the normal viewing speed by a modified VHS video cassette recorder. This system lacks the ability to duplicate a videotape at anything more than twice the normal viewing speed and operates only in the analog domain. This system lacks the ability to write video cassette tapes using parallel data streams of compressed data from a digitally encoded file.
It is most desirable to duplicate a 2 hour tape in about 3 minutes. This requires a 40xc3x97 speed up. It is not possible to just speed up the drum speed and tape advance speed 40xc3x97 on a conventional VCR for a number of reasons including: The drum speed would be 72,000 rpm and the highest signal frequency would be about 200 MHZ, a frequency that would not pass the rotary transformer nor record with conventional heads. Solutions to this problem have been accomplished by some professional xe2x80x9cDxe2x80x9d class recorders wherein parallel heads have been used to keep the head drum speed to reasonable speeds. Typically, such recorders sell for hundreds of thousands of dollars and employ very expensive coaxial, high-frequency rotary magnetic transformers, or coaxial capacitive couplers as described in U.S. Pat. No. 4,870,522 issued in the name of Lelandais.
The recorder of the invention accepts 16 parallel inputs of information; 8 video inputs and 8 high fidelity audio inputs) both appropriately time interlaced, and records them in VHS, PAL, SECAM or other helical scan formats on a cassette tape moving at 40xc3x97 the normal play speed. A decompression engine which converts an MPEG-2 movie data stream into 8 parallel video/audio outputs is the preferred source of signal for the presently disclosed recorder. Such a decompression engine is described in U.S. Provisional Application, Ser. No. 60/013,805, filed Mar. 21, 1996. The preferred tape recorded format is the VHS format, and the present specification discloses the invention in terms of hi-speed recording a tape cassette for playback in a VHS recorder. One skilled in the art following the teachings of the. invention can readily determine the geometrical equivalent changes to a PAL or SECAM helical scan recorder to achieve equivalent high speed recording. The tape recorded by the VHS compatible recorder of the invention is indistinguishable from a tape recorded on a standard VHS VCR; the tape is fully compatible for playback on such a standard VHS VCR. A VHS tape of a movie having a 120 minute play time is recorded by the disclosed 40xc3x97 speed recorder in 3 minutes. The video inputs feed 16 video record heads mounted on a scanner rotating at 5xc3x97 the normal scanner playback rotational frequency, and the high fidelity audio inputs feed 16 audio record heads alternating in position with the video record heads around the periphery of the scanner. The video and audio signals are fed to the rotating heads through a capacitive disk coupler having 16 facially opposed rings with inter-ring shielding. The scanner rotates between an upper cylinder, and a lower cylinder having a helical fence for guiding the tape, and in half a revolution of the scanner 8 fields of video and high fidelity audio information are recorded as 8 slant tracks, with the video recorded on top of its associated audio information. The recording is compatible with playback on a standard VHS VCR operating at normal playback speed.
Integrated circuit head drive amplifiers mounted on the scanner are powered by an ac/dc converter fed by an ac source coupled to the scanner through a rotary transformer.
Rapid acceleration of the tape to 40xc3x97 speed is implemented by means of a capstan-pinch roll assembly cooperative with a tape tension controlled supply reel servo and take up reel servo slaved to the supply reel servo. The 40xc3x97 tape speed mandates a 2.4 MHz bias frequency for conventional audio recording on the VCR audio edge tracks, which is 40 times the normal 60 KHz audio bias frequency. In a VHS VCR the audio bias oscillator is also used as the excitation driving a full tape width erase head, but in the recorder of the invention the corresponding 2.4 MHz audio bias frequency cannot be used as the erase signal as the power dissipation in the erase head rapidly increases with frequency resulting in possible overheating of the erase head and tape. A separate oscillator in the frequency range of from 180 KHz to 400 KHz is used for erasure. Additionally, a tape header containing administrative information may be recorded on the edge audio track using frequency shift keying modulation.
In the NTSC format there are 262.5 H lines per field. The SMPTE specification for VHS recorded video requires at least an additional 9 H lines at the start of each field, so the standard spec requires at least 271.5 H lines per field in a VHS recording. The 40xc3x97 recorder requires the addition of at least 13.5 H lines (rather than just 9 H lines) for a total of 276 H lines per field to be recorded by the recorder of the present invention, as will be explained below.
Accurate tape guiding at the 40xc3x97 speed is critical, and provision is included for adjustment of the tape guides to provide clean, non-amplitude modulated signal envelopes during operation. To accomplish this, two of the scanner record heads are switchable from their record amplifiers to reproduce amplifiers also mounted on the scanner. During test, a test tape is played back and the reproduced signals checked for uniformity and amplitude stability. The guides are then adjusted so that tape tracking results in acceptable signal envelopes.